GASES

Gases are all around us, but although many, such as
perfume, can be smelt, most gases are invisible. Like liquids, gases can flow
but, unlike solids or liquids, gases will not stay where they are put. They
have no set shape or volume, and they expand in every direction to fill
completely whatever container they are put into. If the container has no lid,
the gas escapes.

Gas particles move around at over 1,600 km (1,000 miles) per hour.
The particles are widely spaced and can move freely in any direction. Gases can
spread out to fill whatever container they are put into. When gas particles
collide, the forces between them are not strong enough to keep them together
– instead they bounce apart.

Vapour is a gas that has evaporated from a liquid before the liquid
has reached its boiling point. Water, for example, boils to form a gas at
100°C (212°F). But, even at much lower temperatures, some water
particles escape from the liquid to form a gas, called vapour, that mixes with
the air. When vapour cools slightly, the gas forms droplets seen as mist.

EXPANSION

In hot air balloons, a burner heats the air inside. This causes the
particles of air to gain more energy and so they move faster and farther apart
from one another, pushing at the sides of the balloon. Heat always causes gases
to expand. If you left a balloon near a fire, the air inside could expand so
much that the balloon would burst.

Gases can be easily squashed, or compressed. When you push a bicycle
pump, for example, you are squeezing the air inside into a smaller space. The
air particles are forced closer together, and bang against each other and
against the sides of the pump.

PRESSURE

Why does a champagne cork explode out of a shaken bottle? The
champagne inside the bottle contains lots of tiny bubbles of gas. Shaking the
bottle releases the gas, and the high-speed gas particles bang against the
cork. This creates an enormous pressure on the cork, and eventually forces the
cork out of the bottle.